The present invention relates to a spray applicator and method useful for applying small quantities of a test substance to a test unit with exceptional efficiency by using a combination of ultrasonic, electrostatic and optionally gas-assist technologies. When used for spraying plants or insects, this device facilitates evaluating the herbicidal, fungicidal or insecticidal properties of a test substance in microliter volumes using only microgram quantities of the substance.
Substances useful for the protection of plants have traditionally been discovered by screening test substances for biological activity on intact plants grown in soil in greenhouses or growth chambers. In these assays, test substances (such as chemical compounds or biological organisms) are sprayed using conventional technology (Matthews, G. A., Pesticide Application Methods, 2nd ed., Longman Scientific and Technical, 1992, Ch. 5 (Hydraulic Energy Nozzle), pp. 99-113) either onto test units comprising soil containing ungerminated seeds or onto test units comprising plants (crops or weeds) which may be infected with specific plant pathogens or infested with specific insect pests. The treated test units are cultured for days or weeks after which time the effectiveness of the test substance is assessed. Such bioassays have the advantage of testing the activity of substances under relatively realistic conditions of plant growth stage and infection or infestation using crops and pest species of economic importance. However, they are time-and-space consuming and labor intensive. Furthermore the conventional spray application technology used requires much greater amounts of the test substance (typically 10-60 mg of chemical compounds) than is actually applied to the surface of the soil or plant because of losses. These losses arise from both the dead volume in the applicator and from spray entrained in the air space or overspray reaching the walls of the spray chamber rather than reaching the test unit The requirement for this amount of test substance places constraints on the synthesis or. acquisition of compounds to be tested. This requirement is becoming more difficult to satisfy as modern synthetic methods such as combinatorial chemistry increasingly make very large numbers of chemical analogs available but only in very limited quantities (often less than a milligram each).
An alternative to such large scale testing is the use of in vitro assays or miniaturized in vivo assays. In vitro assays typically assay the affect of a test compound on one or more specific target enzymes or binding proteins, while miniaturized in vivo assays utilize surrogate indicator species (such as Arabadopsis thaliana or yeast) which can be grown in microtiter plates. These assays have the capability of testing very large numbers of compounds using microgram quantities of test compounds such as are produced by combinatorial or other automated synthetic methods. The disadvantage of these assays is that results on isolated enzyme target sites or surrogate indicator species often do not translate to more realistic, macro-level insecticide, fungicide or herbicide assays on the actual pest species and crops of interest. For the objective of crop protection chemical discovery, in vitro assays and miniaturized in vivo assays produce both false positive and negative results.
Thus a means of efficiently applying small amounts such as microgram quantities of test substances to test units (e.g., organisms of agronomic importance) is needed. Furthermore the ideal application method would deliver the test substance as a spray rather than a drench, dip or other method not readily useful for commercial application. The present invention provides an apparatus and method satisfying this need.
This invention pertains to an apparatus for applying a test substance to a test unit, the apparatus comprising:
(a) an atomizer for producing droplets from a solution or suspension containing the test substance; and
(b) a means for electrostatically charging the test unit with charges opposite to that on the droplets to attract the droplets to the test unit.
The apparatus may further comprise an optional gas-assist device to move the atomized droplets towards the test unit.
This invention also pertains to a method for applying a test substance to a test unit, the method comprising:
(a) atomizing a solution or suspension containing the test substance into droplets; and
(b) electrostatically charging the test unit with charges opposite to that on the droplets to attract the droplets to the test unit.
The method may further comprise moving the atomized droplets towards the test unit by use of a gas-assist device.